It is known to provide apparatus for determining the range of one object such as a motor vehicle from another object such as another motor vehicle or a fixed object such as a wall of a building. One form of known apparatus employs ultrasonic radiation to determine the range whilst another form of apparatus employs electromagnetic radiation using a technique known as RADAR (RAdio Detection And Ranging).
In one arrangement a source of ultrasonic radiation is mounted to a bumper (or fender) of a vehicle together with a corresponding detector. The source is arranged to transmit an ultrasonic radiation signal away from the vehicle, the detector being arranged to detect a portion of the ultrasonic radiation signal reflected by an object back towards the vehicle. A controller is arranged to determine a length of time between transmission of the signal and detection of the reflected signal.
It is to be understood that knowledge of the speed at which the signal travels between the vehicle and the reflecting object may be used to determine a distance of the vehicle from the object. In some arrangements a controller may be configured to sound an alarm or apply a brake of the vehicle in the event that a distance of the vehicle from the object becomes less than a prescribed value.
The present inventors have recognized that a problem exists with known apparatus in that erroneous readings of range can be given in certain situations. This is because the position on the object from which the transmitted signal is reflected can change as the vehicle approaches the object.
FIG. 1 illustrates a scenario in which such a change in the position of reflection occurs. In FIG. 1(a) a motor car 5 is shown approaching a lorry 20. A transducer device 10 arranged to transmit an ultrasonic radiation signal 11 ahead of the car 5 is installed in a front bumper 7 of the car 5. The device 10 is also arranged to detect a reflected portion 13 of the transmitted signal 11.
It can be seen from FIG. 1(a) that when the car 5 is relatively far from the lorry 20 the signal is reflected from a rear door surface 22 of the lorry 20. However, as shown in FIG. 1(b), when the car 5 is relatively close to the lorry 20 the signal is no longer reflected from the rear surface 22 but from a rear axle 24 of the lorry 20. The rear axle 24 is further from the car 5 than the rear door surface 22 by a distance d, introducing an error into the measured distance of the car 5 from the lorry 20 corresponding to the distance d. Thus, in the situation illustrated in FIG. 1(b) a controller of the car 5 may fail to warn a driver of the car 5 that a distance between the car 5 and the lorry 20 is dangerously low and that a risk of collision exists.
It is an aim of embodiments of the present invention to at least partially mitigate the disadvantages of known range determination apparatus. Other aims and advantages of the invention will become apparent from the following description, claims and drawings.